Different techniques are known to manufacture resistors by bringing a non-insulating, electrically resistive film or foil material, such as metal film or metal foil, e.g. nickel chromium, cermet film, e.g. tantalum nitride, ruthenium dioxide, bismuth ruthenate, carbon film, or a film of composite material based on a mixture of glass and cermet onto an insulating substrate. In rare cases, the electrically resistive film material may include multiple layers of different of the above named materials. The insulating substrate can be ceramic, silicon, glass or some other synthetic material, and the film material is applied to the substrate by methods such as sputtering (thin film), screen and stencil printing (thick film) or direct printing through a nozzle (thick film). The insulating substrate may have the form of a flat planar sheet or of a cylinder, and accordingly the resistive film is deposited either onto a two-dimensional planar surface or onto a three-dimensional axially symmetric surface. In the voltage divider, both the high and low ohmic resistors are brought onto the same substrate. In addition, highly conductive structures with considerable lower resistivity than the film material of the resistors are deposited on the substrate as well. The highly conductive structures are intended to be used as contacting terminals, and they are placed on the substrate in such a way that the resistive film material of the resistors overlaps partly with them.
In order to achieve voltage ratios of significantly more than unity and at the same time reduce the size of the voltage divider, it is known to arrange the resistive film material of the high ohmic resistor in a long and narrow trace, where the trace is shaped like a meandering form. The term meandering form means that the trace is not just a straight line but curved in such a way that a long length is achieved on a small substrate area. The meandering form may look for example like a square wave, a triangle wave, a sine wave or something more irregular like a serpentine, a zigzag or—in the three-dimensional case—a helical form. This is for example described in U.S. Pat. No. 5,521,576 for thick film resistors and in U.S. Pat. No. 7,079,004 B2 for thin film AC voltage dividers. As is disclosed there as well, the low resistance value of the low ohmic resistor is commonly obtained by arranging the resistive film material in a short and wide trace.
In general, the above described resistive voltage dividers can be used for a wide range of voltage levels, from low over medium up to high voltage applications. While the novel exemplary embodiments of the present disclosure originates from the area of medium voltage sensors, such as the KEVCD and KEVA sensor types by ABB, which are commonly applicable to a voltage range between 3.6 kV and 36 kV, its area of application is not limited to this voltage range.
For medium and high voltage applications of up to one megavolt, voltage ratios of up to several hundred thousand are commonly required in order to step down the quantity which is to be measured to the voltage level of the processing electronics. A possibility to achieve higher voltage ratios is to increase the length of the high ohmic resistor. However, there are limits with respect to the available substrate area and the acceptable size of the voltage divider. It is also possible to reduce the length of the trace of the low ohmic resistor. However, there are technological limitations for the minimum trace length and for the minimum spacing between the connecting terminals. Even though these possibilities exist, it needs to be ascertained that currently, resistive dividers with voltage ratios above twenty thousand are hardly available. Thus, their application in voltage sensors rated for voltage ranges in excess of 36 kV has been limited up to now.